In a statement, Mr Joyce said he's had a "wonderful" time in Parliament.

There has been some speculation Mr Joyce was about to lose his role as finance spokesperson in Mr Bridges' reshuffle.

Mr Joyce says Mr Bridges offered up a high-ranking portfolio.

"Simon has made a very positive proposal to me to stay and contribute as a senior member of the team on the front bench with a choice of portfolio.

"However I feel that it is time for him to get a new team around him to take National forward and win in 2020 and then govern again for the benefit of all New Zealanders."

Newshub understands Mr Joyce stayed with family in Tauranga last night and met with Mr Bridges this morning to inform him he would be stepping down.

In a statement, Mr Joyce said he will remain a supporter of the National Party.

"Personal highlights of my time in office include setting up major infrastructure projects like ultrafast broadband, the major motorway and expressway projects now coming on stream, and the electrification of Auckland's commuter rail network."

His resignation comes after he sent a letter described as 'threatening' to the NBR on Friday, after it published a Matthew Hooton column that was heavily critical of Mr Joyce.

Steven Joyce at Waitangi,
Photo credit: Getty

Mr Joyce's claim of an $11.7 billion hole in Labour's fiscal plan successfully turned the focus to Labour's tight budget during the election. But while economists agreed Labour was running tight fiscals, none came out in explicit agreement with his $11.7B-shaped hole.

Mr Joyce entered Parliament in 2008, becoming Minister of Transport and Minister of Communications the same year.

He became a millionaire before entering politics, founding Radioworks - now known as Mediaworks Radio.

Mr Joyce lives in Albany on a farm with wife Suzanne and their two children.

"Steven has made a huge contribution during his 15 year political career, including in the last decade in Parliament. In that time he has proven an exceptional minister, colleague, advisor and political strategist," Mr Bridges said in a statement.

"He played a major role in rebuilding the National Party, leading the past five elections and helping turn National into New Zealand's largest and most popular political party."

Nicola Willis, National's Wellington Central candidate, will enter Parliament to take up the seat freed upon Mr Joyce's exit.